`PyObject.item()` should follow the JS convention on undefined keys

[mmomtchev]

The [] operator in JavaScript returns undefined if the key is not defined, while the same operator in Python raises an exception.

PyObject.item() should follow the JS convention.

[mmomtchev]

Currently the recommended method to check if a dictionary contains a key before trying to access it is to use the Python in operator from the operator module:

(proxified example)

const op = pymport('operator');

if (op.contains(dict, key).toJS()) dict.item(key);